Nope
MPGe is fair useless and widely misunderstood as the above quotes show, but it is well defined. It is the miles travelled on 33.7 kWh of energy. It is useless because a kWh of electricity is quite different from a kWh of fossil fuel in terms of what it costs, how it is produced, the pollution it causes, and how much work it can perform.
OP: Do any of your friends know their MPG ? I'l guess not. Find out what they do know, and express in the same terms.I had this discussion a couple of years ago with a young girl who was given the old family truck. She was very frugal and she knew how much she spent a week for gasoline but had no idea if that was a lot or a little, or how many miles she drove.
If your friends are typical they will not know how much they drive, how much fuel they buy, or how much it costs them. If that is the case then don't waste your breath because they do not want to know.
Right, I somehow forgot the
third definition of MPGe (energy equivalence). Indeed you are correct, it's fairly useless, and especially so for OP's question.
So there are
three possible meanings to MPGe:
- Energy equivalence: This is the one usually on the sticker, and is a globally true number
- Cost equivalence: The only one that matter in relation to money, and depends on both local gas and electricity prices.
- Emissions equivalence: Matters only for "green" discussions, and is also highly locally variable.
The second is the one I was explaining, since that's what OP was wondering about.
It is not always about you, people have cars with tank sizes other than 14 gallons, your 0.25 gallon variation at the top is a guess, and they do not always start from 1/4 empty or less.
Your calc is fine; it cannot however be relied upon to be particularly accurate.
Back in my ICE age I wanted 0.1% MPG accuracy so I would string fill-ups together and use the same pump. I also knew my total miles driven in the car since I bought it and the total gallons bought, each fill-up to 4 sig digits. Now that was some serious precision for trending, although I never bothered to check my odometer for accuracy.
Sorry to inform, but you never had 4 significant digits. See:
Gas pumps - Measurement Canada (that's Canada, but we generally have very similar rules to the US).
An allowed 0.5% variance in the dispensing system will not give you that many significant figures, and certainly not +-0.1% on that calculation. Still entirely fine for tracking purposes, just perhaps not as precise as you thought.
Supercharging costs vary by state, and e.g. until recently in NM charging was per minute so it was possible to optimize the charging session and gain close to 2.5 kWh a minute. That would work out to less than 2 cents a mile.
I have paid $120 for close to 1000 kWh, so about 4500 miles. The values are from the 'scan my tesla' app and my history file at tesla.com
For
so many reasons, optimising per-minute Supercharger cost is unrealistic (it's even outside of our control sometimes due to station issues). You can only pull 150kW for so long, and there are additional factors.
Especially now that Tesla charges for gross energy delivered like everyone else (not just net energy added to the pack), there are multiple losses to consider: preconditioning, battery heating during charge, and other auxiliary power (climate control, pumps, etc.). Any optimal $/kWh charges prior to this change would no longer be achievable.
Since you mentioned ScanMyTesla specifically, I can almost guarantee you're tracking net energy and not gross energy. You'd have to do some math to calculate the actual energy delivered during the session. So you're probably underestimating true cost, though by how much is unknown since we cannot see into the past.
Looking at my notes, I suggest that 12% is added even for per-kWh stations (though this has a number of variables). In my experience with per-minute stations (in which I
have optimised for charge rate since I was using ABRP), these end up universally costing more per kWh per charge than the actual per-kWh stations in real life travel use-cases.
If the 2000 miles was on a vacation/pleasure trip, then some of the charging could have been free; as in L2 destination charging. It's why I suggested that cost/mi be the main factor of comparing operating efficiency, rather than MPGe.
This is why I linked to the Wiki article, but failed to show how that applies in hindsight. I struggle with it because free charging is definitely a thing, but personally I find I can't use it during trips (contrary to suggestions in the community, I do not plan trips around free overnight charging - they often require staying at more expensive places anyways, so is it really free?).
If we use this formula:
You can substitute some known values assuming a new Model 3 LR AWD. Total usable capacity would be about 74.1kWh for 322 rated miles. If you are using/charging 80% (e.g. 10-90%), that's 59.3kWh. Now, we just have to figure out the
actual $/kWh rate.
I will take an example in my history: $0.27/kWh in Colorado.
This is mostly trivial to calculate, but in my experience you'll have the following:
- About 2kWh for preconditioning on the way
- About 4kWh for heating while charging
- About 1kWh for other various things (climate control, pumps, computers)
So to deliver the 59.3kWh needed to charge out LR pack by 80%, we actually need about 66.3kWh delivered during the charge. This works out to
$17.90, and a rate equivalent of
$0.302/kWh net. That's about 12% over the $0.27/kWh otherwise advertised.
Now, that was for 80% of the range, so 257.6 rated miles. Just found $1.73/gal for Denver. Now, let's plug that all into the formula!
MPGe = (( $1.73/gal / $0.302/kWh ) / (59.3kWh) ) x 257.6
MPGe = 24.9
Of course, this is assuming rated conditions. Travelling faster or in winter would consume rated miles faster than miles actually traveled, so the MPGe would be worse.
This also calculates out to $0.0695/mi.
This question got me thinking... and after looking around the forum I realized what's going on. OP posted this recently:
2000 Mile Roadtrip from LA to Colorado. First long road trip (Pics)
If my speed reading is still up to par, several stops (charges) were made at L2 destination chargers, which I will assume were free. That would explain the $$$ difference.
Oh dang, so it turns out my Colorado charging example is actually especially relevant to OP. Nice! The only overnight charging I got was on my way back through Washington (by accident actually, the chargers weren't on PlugShare yet and the hotel didn't advertise them).
Indeed, $85 for 2000 miles isn't super representative due to the free charging. Definitely closer to $140 would be realistic, had it all been Supercharging. The truth, of course, is somewhere in between (to account for both home charging and free charging).
